Conventionally, in the signal processing of a MOD, an IC called read channel has been used. Key 111 in FIG. 4 indicates a read channel. This read channel 111 comprises an amplifier 102 and a low-pass filter 103.
When information in a MOD is read out, a photo pickup, which is not illustrated, generates a photo-current that corresponds to the information written in the MOD, and the photo-current is first converted to a voltage by an I-V amplifier 110, and then is output to the amplifier 102 in the read channel 111. This voltage is amplified by the amplifier 102, has a waveform reproduced with noise in the high band removed by the low-pass filter 103, and then is output to the signal processing circuit 104 in a later stage.
For this low-pass filter 103, a linear phase filter such as a Bessel filter or a ripple filter is used. The configuration is such that group delay characteristics will be flat within the pass band of the filter, and such that no distortion will be generated in the output waveform. Therefore, by inputting a pulse signal in the low-pass filter 103 and measuring the delay time, a cutoff frequency can be obtained.
In this manner, with a low-pass filter incorporated in an IC, the cutoff frequency would vary due to a variation in each element. Thus, in the prior art, the delay time and the frequency characteristics of each low-pass filter were measured before the IC chip was sealed in a package, and an appropriate cutoff frequency was set for each element by cutting off the fuse or in some other way.
However, as the delay time and the frequency characteristics were measured by a probe placed in contact with the terminal of each IC chip, the following problems existed: errors due to noise were significantly large; and when the IC chip was sealed in the package after completing the measurement, characteristics such as the delay time and the frequency characteristics would change from the measured characteristics because of stress at the time of sealing, thus the set cutoff frequency would no longer have an appropriate value.
The present invention was created to resolve the aforementioned problems of the prior art. The object is to offer a filter circuit with which an appropriate cutoff frequency can be automatically set at each element.